Caloric vestibular stimulation (CVS) has been widely and safely utilized for more than a century for diagnostic purposes, particularly in the emergency room to detect brain function after trauma. CVS activates the sensory organs of the vestibular system (VS) located within the inner ear. The core elements consist of the semi-circular canals, which sense rotational motion, and the otoliths, which sense linear acceleration. Motion within the semi-circular canals is detected through motion of internal fluid (endolymph), which in turn activates hair cells that generate electrical signals, which are then transmitted via the 8th cranial nerve to the brainstem and widely throughout the cerebellum and cortical regions. In CVS, irrigation of the outer ear canal with warm or cold water changes the density of the endolymph in the semi-circular canal of the inner ear, which in turn activates the pathways noted above. Nystagmus, or the vestibulo-ocular reflex, is an easily observed result of CVS where the eyes move spontaneously, even if the patient is unconscious. See generally L. Patten, Vestibulo-ocular reflex paths, Br. J. Ophthalmol. 16, 257 (1932).
There have been intriguing (but largely anecdotal) reports of using CVS as a therapeutic measure. See generally L. Rogers and L. Smith, PCT Appl. Pub. No. WO 2009/020862. Survey articles document a variety of outcomes and discuss some of the mechanisms involved (Miller et al., Acta Neuropsychiatria 19:183-203 (2007); Been et al., Brain Research Reviews 56 346-36 (2007)). Squirting or blowing warm/cold water/air into a patient's ear, however, is crude, does not provide closely controlled thermal activity, and is not consistent with medical dosing.
CVS is known to activate specific brainstem, cerebellar and cortical sites, which have therapeutic potential, as demonstrated through functional imaging (Bottini et al., Exp Brain Res 99: 164-169 (1994); Bense et al., Ann NY Acad Science 1004: 440-445 (2003); Dieterich et al., Brain, 131, 2538-2552 (2008); Hum Brain Mapp (September 2009); Naito, Cortical correlates of vestibulo-ocular reflex modulation: A PET study. Brain 126 (2003)).
In addition, vestibular stimulation is also known to release important neurotransmitters (e.g., serotonin, acetylcholine, histamine, endorphins, vasopressin and dopamine) (MA Fu-rong et al., Chin Med J 120(2):120-124 (2007); Horii et al., J. Neurophysiol. 72, 605-611 (1994); Tabet, Age and Aging, 35: 336-338 (2006); Horii et al., J Neurophysiol 70 1822-1826 (1993); Horii et al., Brain Research 914: 179-184 (2001)).
In contrast to both pharmaceutical treatment and neurostimulation devices which employ electrical signals, CVS appears to have an advantage: although nystagmus habituates with repetition of CVS (Naito et al., supra (2003)), the vestibular neurological response appears not to be subject to such habituation or accommodation. (Emani-Nouri, Acta Otolaryngologica 76, 183-189 (1973)). In addition, CVS does not have the potential for side effects in the same manner that a pharmaceutical does. Yet, CVS has not attained wide-spread use for therapeutic purposes. Hence, there remains a need for methods to utilize caloric vestibular stimulation for therapeutic purposes.